Comparing the brightness of the left zone with the right zone can be difficult, particularly when testing long focus mirrors. The longer the focal length the less contrast.
The radius of the mirror center is important, and harder to measure than the radius at the mirror edge.
Some mirror makers prefer a cardboard mask to mask the zones.
To get these pictures with the knife edge near the radius of curvature of the mirror center, the 70 percent zone and the mirror edge we used an already completed and coated mirror and traced thought the mask onto the coating with a marker pen. Normally the mirror would be uncoated during these tests.
Near the 70 percent zone.
In this animation of pictures the knife edge is on the left and moves left, out of the light, and then returns into the light again.
The shadow is on the right because the knife edge is outside of focus.